Fiber optic communications systems, such as those used pervasively in telecommunications, are comprised of three basic components: a transmitter/light-source, an optical fiber link or channel, and a detector/receiver. Such systems gain most of their advantage with regard to data transmission capacity, speed and distance if a laser—as opposed to a light-emitting diode (LED)—is used as the light source; and accrue further benefits with respect to cost, compactness, reliability, and power consumption if a semiconductor laser diode in particular is used as the transmitter light source.
Although a laser is nominally a monochromatic light source, a laser will in fact generally produce light at several emission wavelengths (referred to as modes), unless steps are taken to suppress all but one of the modes, in which case the laser is referred to as a single-mode laser. Such single-mode lasers provide superior performance in optical fiber systems on account of reduced dispersion losses, which in turn permits higher data rates and longer transmission distances.
The transmission capacity and functionality of a fiber optic link can be increased considerably by the technique of wavelength-division multiplexing (WDM). In general, multiplexing refers to the simultaneous transmission of several signals or messages on the same circuit or channel. For example, in coaxial cable systems frequency-division multiplexing is realized by providing several independent carrier signals, each with a distinct frequency assignment, and each modulated with an independent message signal. At the receiving end of the coaxial cable, selective bandbass filters separate the several carrier frequencies so that each carrier can be demodulated to yield the original message signals. As another example, in digital data transmission systems time-division multiplexing is effected by interleaving bit streams from several sources to form a composite high-rate bit stream. At the receiving end, the bit streams can be demixed with proper considerations of time frames and synchronizations.
An analogous multiplexing technique, called wavelength division multiplexing (WDM), is used in fiber optic communications systems. WDM is based on simultaneous transmission of light signals that are allocated to different carrier wavelengths. Several lasers with discrete and well-separated emission wavelengths are independently modulated by several message signals. The allocated carrier wavelengths of a WDM system are referred to as ‘channels’. The modulated laser outputs are all launched into a common optical fiber, and are de-multiplexed at the receiving end of the link by wavelength-sensitive filters. Signals thus separated according to their carrier wavelength are coupled to detectors dedicated to a particular channel wavelength assignment. Fiber optic systems employing WDM are now a well-established part of the telecommunications infrastructure.
A wavelength division multiplexing system requires at least several—sometimes as many as 50 to 100—laser transmitters, each operating at a distinct emission wavelength. Since the emission wavelength is, to a large extent, an intrinsic property of a conventional laser, a WDM system would seemingly require several or more different types of laser diodes, each with a specified emission wavelength. On the contrary, the preferred implementation of WDM systems is to instead use just one type of laser, but one which can be readily adjusted and set by the user to operate at any of several prescribed available emission wavelengths. Such lasers are referred to as ‘tunable’ in that the emission wavelength according to the specific application and system requirements can be adjusted in the field. In WDM fiber optics systems with multiple channels, each channel transmitter would use the same type of laser, but with its singular emission wavelength tuned to and set for the wavelength allocated for that particular channel. This approach, namely, employing just one kind of tunable laser rather than many different kinds of single-mode lasers with unique emission wavelengths, addresses what is commonly known as the provisioning problem, as it considerably simplifies the inventory, deployment, assembly, and maintenance of a WDM system. For example, rather than stock, install, and maintain many different types of lasers, a generic tunable laser is used for all transmitter light sources, and its emission wavelength is selected according to where it is inserted into the WDM system. The use of tunable laser diodes also facilitates reconfiguration of WDM systems, as transmitters can be readily re-assigned to new channels by selecting a new emission wavelength. Hence, providing an apparatus to realize mode switching of a compact external cavity laser, and especially one of simplified construction, is useful for WDM optical communications systems would provide an important advance in the field of laser technology.